Superconducting Detector and Method of Making Same

This invention relates to superconductive materials and the use of such materials as infrared sensors. Metal-oxide ceramic superconductive materials were discovered circa 1987. Such materials conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen (77K or -196.degree. C.).

Materials exhibiting this property are called high T.sub.c superconductors, where T.sub.c is the transition temperature at or below which superconductivity is exhibited. A particularly useful high T.sub.c, superconductor is a yttrium-barium-copper oxide, such as YBa.sub.2 Cu.sub.3 C.sub.x, where x is 4 to 8 and preferably 7, which has a T.sub.c, of 90K or higher.

High T.sub.c superconductors have found many applications in industry, since they can be cooled by Liquid Nitrogen (LN), instead of the awkward and expensive liquid helium used in the past. These applications include, for example, Josephson devices, magnetic detectors formed of superconducting quantum interference devices (SQUIDS) and bolometers.

Bolometers, or radiation sensors, have heretofore been fabricated of high T.sub.c material. (See "High T.sub.c Superconducting Infrared Bolometric Detector", B.E. Cole, SPIE Vol. 1394. "Progress in High-Temperature Superconducting Transistors and Other Devices", (1990), pp. 126-138. A bolometer converts absorbed energy to a temperature rise and detects that temperature rise with a thermally sensitive element, such as a resistor The highest bolometer sensitivity is achieved by selecting detector materials with the highest temperature coefficient of resistance (TCR) and low resistivity. Metals have a low TCR in the range of 0.002/.degree. C. to 0.003/.degree. C. at room temperature and a somewhat higher TCR at (LN) temperatures. Semiconductors can have a much higher TCR, but have a very high resistance. High T.sub.c materials have TCRs in the order of 0.5/.degree. C. and the change in resistance at the transition edge is extremely abrupt, making them excellent candidates for bolometric devices.

Another factor of importance in bolometer performance is the thermal properties of the sensor. Both thermal mass and thermal conductance should be minimized within constraints of proper thermal time constant.

From the above, it may be seen that the sensitivity of radiation detectors is limited by the device operating temperature. Presently, room temperature detectors are in wide use while liquid helium devices are reserved for the most demanding applications. For radiation detection in the near IR spectrum, i.e., below about 20 .mu.m, LN detectors offer better performance than room temperature detectors and lower cost than helium-cooled devices LN-cooled detectors for wavelengths longer than 20 .mu.m are presently not available. Infrared detectors detect radiation by either a thermal process or a photon process. In detectors employing a photon process, incident radiation excites a carrier from a valence state into a conduction state. Sensitive semiconductor detectors relying on this process are used to detect near infrared radiation. Photons at longer wavelengths have lower energies and can only excite small energy transitions. But low energy transitions can also be excited by thermal energy. As a result, photon detection devices become much less sensitive at wavelengths beyond 20 microns and are drowned out by thermal energy. Such devices only work well if cooled at liquid helium temperatures The High -Tc detector, however, exhibits high performance at the critical temperature and can be used at longer wavelengths where the photon process detectors (i.e., MCT, photovoltaic, or photoconductive detectors) lose sensitivity.

Accordingly, a need exists for a radiation detector that will provide an LN-cooled alternative for high performance infrared radiation detection at longer wavelengths i.e., in the range of 20 .mu.m or higher.

An improved radiation detector, particularly suitable for IR detection, is comprised of a sensor on a flexible membrane extending over a gap formed in a base body. The membrane is part of a monolithic structure, including an epitaxial silicon device substrate. The membrane is formed as a thin film of Si.sub.3 N.sub.4 (silicon nitride), which is deposited on the device substrate. The device substrate is then bonded to the base body with the membrane extending over the gap. The bulk of the device substrate is removed leaving only a thin film of epitaxial silicon formed into a device body. The device body is centrally disposed integral with the membrane and adjacent the gap and thermally isolated thereby from the base body.

An epitaxial buffer layer is grown on the silicon device body and a high T.sub.c superconductive epitaxial layer, preferably of YBa.sub.2 Cu.sub.3 O.sub.x is formed on the buffer layer and patterned into a resistor to form the IR sensor. Contact metallization is then formed on the superconductive layer to complete the detector.

The detector of the invention achieves superior performance with the combination of effective thermal isolation, low thermal capacitance, and a high quality epitaxial superconducting film. From theoretical calculations, it is predicted that an optical noise equivalent power (NEP) of 4.times.10.sup.-12 W/Hz.sup.178 can be achieved with the detector of the invention, compared with 5.times.10.sup.-10 W/Hz.sup.1/2 typical for pyroelectric detectors [Richards, P.L. et al., "Feasibility of the High T.sub.c Superconducting Bolometer ", Applied Physics Letters, p. 283, Vol. 54(3)] Responsivity is estimated at 750 V/W. This detector also has a fast response time, due to the low thermal capacitance. The time constant is estimated at 40 .mu.s. For comparison, the high-T.sub.c bolometer reported by Verghese and Richards (Verghese, S. et al., "Fabrication of an Infrared Bolometer with a High T.sub.c Superconducting Thermometer", IEEE Transactions on Magnetics, Vol. 27, p. 3077) achieves an optical NEP of 2.4.times.10.sup.-11 W/Hz.sup.1/2, a time constant of 55 ms, and a responsivity of 17 V/W. The microbolometer reported by Nahum, Hu and Richards (Nahum, M. et al., "Fabrication and Measurement of High T.sub.c, Superconducting Microbolometers, IEEE Transactions on Magnetics, Vol. 27, p. 3081) has a comparable responsivity (478 V/W), and the electrical NEP is 4.5.times.10.sup.-12 W/Hz.sup.1/2. The optical NEP, however, is much higher in Nahum et al. because most of the incident radiation is absorbed by the thick substrate. A time constant of 50 .mu.s is calculated from information in Nahum et als. report.

FIG. 1 is a plan view of a preferred embodiment of a detector of the invention.

FIG. 2 is an enlarged view of a schematic section through the detector.

FIGS. 3A-3E is a series of schematic sectional views showing important steps in fabricating the detector of the invention.

FIG. 4 is a plan view of an alternate embodiment of a smaller device of the invention which is partially cut-away to show the beam struts underlying the gold leads 22'.

Referring now to FIGS. 1 and 2 it may be seen that the detector 100 is comprised of a low thermal capacitance epitaxial silicon device body 20 on which a sensor is formed of a thin film of high Tc material 30, preferably Y Ba.sub.2 Co.sub.3 O.sub.7, which is epitaxially grown on an optional buffer layer 32 of epitaxial yttria-stabilized zirconia grown on the device body 20. The film 30 is patterned into a convoluted resistor shape to form the sensor. Electrical leads 22 extend from the sensor 30 to the perimeter of the detector 100. The Diameter D of the detector is about 500 microns, the radial length R of the leads 22 is about 215 microns the length L and width W of the sensor 30 is about 50 microns and the thickness T of the leads 22 is about 5 microns. The sensor body 20 is suspended by a thin membrane 16, preferably formed of silicon nitride, across an air gap formed in a Si base body 18. Membrane 16 provides a low thermal conduction path from the device body 20 and isolates the sensor 30 from its environment.

One of the problems overcome by the present invention is finding a way to form an epitaxially grown sensor on a membrane so as to produce a high quality sensor which is thermally isolated. This is made possible by the method described below with the aid of FIGS. 3A-3E.

A single crystal silicon device substrate 10 is provided. Substrate 10 preferably is doped by in-situ doping of boron atoms during crystal growth to form an etch back layer 12 for subsequent processing leaving a region 14 of epitaxial Si beneath etch back layer 12. A thin film (about 1500.ANG.) of silicon nitride, or other suitable membrane material is deposited on region 14 to form membrane 16. Membrane 16 may be deposited directly on region 14 preferably by Low Pressure Chemical Vapor Deposition (LPCVD) Optionally a stress relief layer of SiO.sub.2 (not shown) may be formed in advance of membrane 16 (FIG. 3A).

A base body 18 of silicon is also provided. Body 18 is etched using an SF.sub.6 plasma or KOH process, as shown, to form a generally concave cavity 24 in an otherwise planar upper surface thereof (FIG. 3B).

Next, device substrate 120 is bonded to base body so that an air gap 26 is formed beneath membrane 16. In the bonding process, the surfaces of the substrate 10 and base body 18 to be bonded are prepared using a well known standard cleaning procedure developed by RCA, and the surfaces are brought into contact. The contacted hydrated surfaces bond by hydrogen bonding. They simply stick together when placed in contact. The bonded pairs are then annealed in a furnace for one hour. The hydrogen diffuses out, and the elevated temperature causes the two surfaces to chemically bond and fuse together. The anneal is performed at 800.degree.-1100.degree. C. in a N2 gas ambient. A lower temperature, 800.degree. or 900.degree. C. is preferred. Higher temperatures produce a better bond but also reduce the etch-stop layer concentration due to diffusion. (FIG. 3C.)

The bulk of silicon substrate 10 is then removed by etching back the Si material down to the etch back layer 12 using a suitable etchant such as potassium hydroxide (KOH). Layer 12 is also removed by an 8:3:1 solution of acetic acid, nitric acid and hydrofluoric acid. Si material on either side of dotted lines 27 of FIG. 3C is removed down to Si.sub.3 N.sub.4 membrane 16, using well known photolithography techniques leaving single crystal Si device body 20 integral with membrane 16 and centered over gap 26.

A yttria-stabilized zirconia (YSZ) buffer layer 32, about 700.ANG. thick, is epitaxially grown on the device body 20 preferably by a laser ablation process followed by growth of a YBa.sub.2 Co.sub.3 O.sub.7 (YBCO) film 30 , about 500.ANG. thick, likewise by laser ablation. The film 30 is patterned using a weak organic acid, such as EDTA, into a convoluted resistor shape. Ion milling could also be used for patterning. Finally gold is deposited and patterned to provide electrical contact leads 22. (FIG. 3E)

In the laser ablation process, the substrate 10 is placed in a vacuum chamber at 800.degree. C. in an oxygen ambient. The chamber contains targets which are struck and ablated by laser pulses. The ablated material forms the desired thin film on the substrate 10.

The resultant thermal capacitance of the device body 320 is 4.times.10.sup.-10 J/K and the thermal conductance from the sensor 30 to the base body 18 is estimated at 1.times.10.sup.-5 W/K during cryogenic operation. The resistance of the sensor 30 is 500 ohms.

This completes the description of the preferred embodiment of the invention. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation many equivalents to the specific embodiments of the invention described herein. These and all other equivalents are intended to be encompassed by the following claims.

For example, the device could be made smaller by patterning the silicon nitride film 16 so that instead of a large area membrane, the detecting element is suspended by a smaller membrane 16' and short silicon nitride beams 160 as in FIG. 4 (as in the Honeywell device reported by Cole supra). This would require the use of silicon-rich silicon nitride, which has lower stress and is more suitable for patterned structures. This may achieve a comparable or better thermal isolation as well as a smaller total device area which is important for array application but requires an additional mask step. Note: FIG. 4 employs like numerals for corresponding items in FIG. 1 with a prime suffix.

Another alternative is to deposit and pattern an absorbing layer (such as gold black) on top of the silicon nitride layer 16 prior to bonding. Then, the finished device would have an absorbing layer underneath the detecting element to improve the absorption and thereby the optical efficiency of the device Adding this process may make bonding more difficult, because the additional process step may degrade the quality of the bonding surface, and bonding would have to be done at a lower temperature in order to protect the absorbing film. Note: electrostatically assisted bonding can be performed at 400.degree. C.

Other materials may be better suited than silicon for use as a substrate for high quality YBCO films (for example, MgO, LaAlO.sub.3, or SrTiO.sub.2), however, processing techniques for such materials are not as well developed. Note that processing of superconducting films is difficult because the superconducting films are easily degraded, for example, they react with water.

There are different equivalent approaches to patterning the resister sensor 30 from the superconducting film. One approach is to pattern the buffer layer 32 instead of the superconducting layer 30, in which case a superconductor would only be formed over the buffer layer regions. Alternatively, the silicon structure 20 could be patterned into a resistor geometry. If the silicon structure 20 is thick enough and has steep walls, then the superconductor 30 would be isolated from the field and the resistor would be formed without having to pattern the superconductor.

Another alternative embodiment to this process would be the addition of a passivation layer (not shown) to protect the completed device. Such a layer may be necessary for packaging in commercial production, but would add to the thermal conductance.

Finally, since the detector is silicon based, readout electronics can be integrated on the same chip as the sensor.